Extensively used processes are known for the vulcanization of large rubber sheets and belts. According to a solution the rubber sheet together with the fabric for preventing the layers sticking to each other, is wound on a steel drum, then the wound up material is heated in boiler with steam or hot air, and is then vulcanized.
The drawback of that process is that the vulcanization takes relatively long time, because the dissipation of heat transfer takes places from outside inward, and the heat passes to the central threads of the coil only at a slow rate by means of thermal conduction. The degree of vulcanization of the different layers varies, detrimentally influencing the characteristics of the rubber. The poor thermal conduction and the varying vulcanization of the layers determine the coil thickness on the drum, i.e. the length of the rubber sheet. Thus this type of vulcanization of several hundred meter long rubber sheet is out of the question. The vulcanizing boilers are generally pressure tight vessels with steel wall, becoming heated in the process of vulcanization which results in substantial heat loss. Surface of the accompanying fabric becomes indented, thereby producing similar indentations on the surface of the rubber.
Two processes have been generally used for vulcanization of rubber belt. In one of them the vulcanization takes place between pressing plates. These are compressed with hydraulic cylinders during the process of vulcanization. Length of the pressing plates is max. of 15 m, being the same length of the belt vulcanized in a single cycle. Consequently the vulcanization of several hundred meter long belt takes a very long time.
The appropriate degree of vulcanization of the belt-zones at the end of the pressing plates is problematic in this process. Separate tensioning unit is required during the vulcanization process for tensioning the belt section to be vulcanized.
In the other known process the vulcanization is not intermittent but continuous. Here the belt is pressed onto a slowly rotating heated metal cylinder with a pressing band. The cylinder must rotate at a very slow rate, because the belt should be vulcanized while it is in contact with the cylinder. Hence the vulcanization of several hundred meter long belt takes a very long time. Such a process is described in the German Federal Republic Patent No. 2,856,646.
A drawback of both processes in addition to those mentioned above is the considerable heat loss. The heat losses are due to the fact, that not only the rubber materials to be vulcanizedhave to be also heated, but also the bulky machine parts.
So far no process is known which would vulcanize several hundred meter long belts or rubber sheets in a single step in a short time, with low heat loss.